ABSTRACT. Tobacco use remains the leading cause of preventable death in the U.S., responsible for more than 440,000 deaths annually, of which 41,000 are attributable to environmental tobacco smoke (ETS). The federal Department of Housing and Urban Development (HUD) recently passed a rule that will require all public housing agencies (PHAs) to implement smoke free policies in their developments. There is a strong rationale for the new housing policy. Compared to the general public, residents in public housing are at excess risk for ETS exposure for two reasons: (1) Despite overall declines in smoking over time, major disparities persist by income level, education and race/ethnicity. Thus smoking rates among residents living in public housing, a predominantly minority, lower income population, tend to be higher than in the general population. (2) Most public housing residents live in multiunit housing (MUH), an environment that places all residents at elevated risk for involuntary ETS exposure due to smoke accumulation compared to residents living in detached housing. While prohibition of smoking in homes has promising implications for improving health, initiatives to mandate smoke-free housing are still nascent. No empirical studies have objectively evaluated the impact on health outcomes among residents. Estimated health consequences of such policies are currently based on mathematical models, and only a few studies have measured change in resident ETS exposure before and after implementation of housing-wide bans. These studies have mostly been small, with short follow-up periods, and they have given only limited attention to studying the implementation process. To fill this gap, we propose a large-scale natural experiment to examine the impact of the HUD policy on reducing ETS exposure and improving health outcomes. The New York City Housing Authority (NYCHA) is the largest housing authority in the United States, with more than 400,000 residents. Using a quasi-experimental longitudinal design, we plan to monitor ETS before and after policy implementation (tracking every 6 months for 2.5 years post-policy) in 200 apartments of non-smokers and common areas of 10 select high-rise NYCHA buildings, and compare to ETS changes in 200 apartments in 10 demographically matched high-rise buildings in another housing program, not subject to the smoke-free policy. Using geocoded administrative data, citywide NYCHA resident health outcomes will be compared to residents in matched low-income census blocks. Finally we will use survey data, qualitative interviews and focus groups to explore multilevel factors that influence the implementation process and outcomes. This study leverages a strong, ongoing partnership of academic and municipal government investigators, including experts at New York University School of Medicine, senior staff at NYCHA, and leaders from the NYC Department of Health and Mental Hygiene. Findings will be used to develop guidance for implementing SFH policies in public or private MUH settings nationally.